Fuel Types and Quality

Fuel Types and Quality

Fuels

A substance that can supply energy either alone or by combining with another substance is known as fuel. Heat content of fuel is measured in calories.

An ideal fuel:

• Has high calorific value

• Is cheap and easily available

• It is easy to store & transport

• Can be easily controlled

• Has high ignition temperature

• Burns completely without leaving any residue

• The quantity of fuel is expressed in the form of calorific value.

Calorific Value:

• The total quantity of heat liberated by the combustion of a unit mass of fuel in air or oxygen.

• The calorific value of fuels is expressed in kcal/m³ or British Thermal Units (BTU) per cubic foot.

1 kcal/m³ = 0.107 BTU/ft³

Fuel may be solid (e.g., wood, coal, etc.), liquid (e.g., kerosene oil, petroleum, alcohol, etc.), or gas (e.g., producer gas, coal gas, oil gas, natural gas, LPG, etc.). However, gaseous fuels are considered ideal.

Water Gas (Syn Gas):

• A mixture of carbon monoxide and hydrogen.

• Obtained by the action of steam on red-hot coke at 1000°C.

• C + H₂O → CO + H₂ + 28 kcal

Producer Gas:

• A mixture of CO and N₂.

• Prepared by burning coke in a limited supply of air.

• It is the cheapest among gaseous fuels; however, its calorific value is not very high, as it has a large proportion of nitrogen.

Coal Gas:

• A mixture of H₂, CH₄, CO, and other gases like N₂, CO₂, H₂O, etc.

• Obtained by destructive distillation of coal at about 1000°C.

Oil &Gas:

• A mixture of H₂, CH₄, C₂H₆, CO, and other gases like CO₂.

• Obtained by thermal cracking of kerosene oil.

• Used in laboratories.

Gobar Gas:

• Contains CH₄, CO, and H₂.

• Produced by fermentation of gobar (animal dung) in the absence of air.

• Used as a domestic fuel in villages.

Natural Gas:

• A mixture of gaseous hydrocarbons, viz., methane (85%), ethane, propane, butane, etc.

• Liquefied petroleum is mainly butane and isobutane.

LPG and CNG (Petroleum Gases)

LPG (Liquefied Petroleum Gas):

• The petroleum gas liquefied under pressure is called liquefied petroleum gas.

• It is a mixture of butane and propane and is easily compressed under small pressures and stored in iron cylinders. It is used as fuel for cooking.

  • 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O + Heat

CNG (Compressed Natural Gas):

• The natural gas compressed at very high pressure is called compressed natural gas (CNG). It consists mainly of methane (CH₄), which is a relatively unreactive hydrocarbon and makes nearly complete combustion possible.

• The other 5% is made of various gases such as ethane, propane, butane, including small amounts of other gases like N₂, CO₂, H₂S, water vapour, etc.

• The CNG is now being used as a better fuel than gasoline for running buses, cars, and three-wheelers in metropolitan cities (like Delhi). Because of its complete combustion and no unburnt carbon, it is released into the atmosphere to cause air pollution.

Knocking and Octane Number:

Knocking:

• The metallic sound produced due to irregular burning of the fuel is known as knocking.

• The knocking lowers the efficiency of the engine and results in the loss of energy.

• A fuel which has minimum knocking property is always preferred.

• It has been observed that the straight-chain aliphatic hydrocarbons have a higher tendency to knock, while branched or unsaturated hydrocarbons have less tendency to knock.

Octane Number:

• To indicate the quality of gasoline (petrol), a method of gradation has been introduced which is termed 'octane rating' or 'octane number'.

• Two compounds, heptane and iso-octane, have been taken as standard.

• Heptane, which causes maximum knocking, is assigned to octane number zero, and iso-octane, which causes minimum knocking, is assigned the octane number 100.

  • Heptane: C₇H₁₆ (Octane number = 0)

  • Iso-Octane: C₈H₁₈ (Octane number = 100)

Antiknock Compounds:

• To reduce the knocking property or to improve the octane number of a fuel, certain chemicals are added. These are called antiknock compounds. TEL (tetraethyl lead) is the best antiknock compound.

Cetane Number:

• The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a mixture of cetane and α-methyl naphthalene.

• Hexadecane has been assigned a cetane number of 100, while α-methyl naphthalene is assigned a zero cetane number.

• The diesel oil having a cetane number of 75 would have the same ignition property as a mixture of 75% cetane and 25% α-methyl naphthalene.

Flash Point:

• The lowest temperature at which an oil gives sufficient vapours to form an explosive mixture with air is known as the flash point of the oil.

• The flashpoint in India is fixed at 44°C.

Coal:

• On the basis of carbon % and calorific value, there are four types of coal:

Final Thoughts

Fuel is a substance that can provide energy, either alone or by reacting with another substance. Its energy content is measured as calorific value, expressed in kcal/m³ or B.T.U./ft³. Ideal fuels are easily available, cheap, safe to store, and burn completely with high ignition temperatures.

Fuels may be solid (coal, wood), liquid (petroleum, kerosene), or gaseous (natural gas, LPG), with gaseous fuels considered most efficient. Gaseous fuels include water gas, producer gas, coal gas, oil gas, gobar gas, and natural gas, each with unique compositions and uses.

Gasoline quality is measured by octane number, while diesel quality is indicated by cetane number. Coal is classified from low-grade peat to superior anthracite based on carbon content and calorific value.